1. Field of the Invention
The present invention relates to a method of ion implantation into a semiconductor substrate provided with an insulating film.
2. Description of the Prior Art
Ion implantation is an old technique for surface doping of semiconductors and consists essentially of introducing atoms in the surface layer of the semiconductor body by bombardment of the semiconductor body with ions in the KeV to MeV ranges. Ion implantation exhibits the following advantages of an externally controlled, non-equilibrium process.
1. The deposition rate of ions can be varied widely by varying a beam current, for example, from 6.times.10.sup.9 ions/sec to 6.times.10.sup.16 ions/sec.
2. By continuously varying the energy of the ion beam, it is possible to precisely adjust the dopant profile in the semiconductor body.
3. The dosing amount is precisely determined by the product of the beam current and implantation time period.
It is well known in the field of semiconductor engineering that ions are selectively implanted, by using masks of a gate electrode and a thick insulating film, into a semiconductor substrate of semiconductor devices, such as an MOS transistor having a gate insulating film. When a source region and a drain region of the MOS transistor are produced in the process of producing an IC, the gate electrode and the thick insulating film, such as field insulation film, are already formed on the semiconductor substrate. Therefore, the ions must be implanted into the gate electrode and the thick insulating film so that a high dosing amount, which is usually necessary for the source and drain regions, is also imparted to the gate electrode and the thick field insulating film. The electric charges of the ions are, therefore, also imparted to the electrode and film. Since the dosing amount is proportional to the product of the beam current and the implantation time period, either the beam current or the implantation time period must be large in order to provide the necessary high dosing amount. When the implantation time period is long, it is obvious that the efficiency of the ion implantation process is disadvantageously low. On the other hand, when the beam current is high, the insulating film may be destroyed due to the electric discharge of the electric charges between the gate electrode or the surface of the insulating film and the semiconductor substrate. In addition to the gate insulating film, other films, such as a passivation film, particularly a thin portion of a passivation film, may be destroyed due to electric discharges, which are similar to those between the gate electrode and semiconductor substrate.